When exposed to heat and light, a chlorine-containing polymer such as a vinyl chloride resin permits hydrogen chloride to be removed from the molecular chains thereof, resulting in the occurrence of decomposition, discoloration and drop of physical properties. It has been known that the causes of discoloration and drop of physical properties are due to the formation of a polyene structure. To stabilize the vinyl chloride resin against the heat decomposition, there have heretofore been proposed and widely used various kinds of stabilizers or stabilizer compositions.
As stabilizers for chlorine-containing polymers, there have also long been known to use hydroxides or silicates of alkaline earth metals and organotin stabilizers, as well as organotin stabilizers and other inorganic stabilizers in combination.
Japanese Unexamined Patent Publication (Kokai) No. 102770/1974 discloses a chlorine-containing high molecular compound composition having a hydrogen chloride gas-trapping property obtained by adding, to a chlorine-containing high molecular compound, a calcium hydroxide in an amount of not smaller than one-half the amount of chlorine contained in the high molecular compound, and kneading them together.
Japanese Unexamined Patent Publication (Kokai) No. 97648/1979 discloses a halogen-containing resin composition obtained by blending 100 parts of a halogen-containing resin with (a) from 0.1 to 2.5 parts of a basic magnesium carbonate, (b) from 0.1 to 5 parts of one or two or more compounds selected from alkaline earth metal salts and zinc salts of organic acids, (c) from 0.1 to 5 parts of a dipentaerythritol, (d) from 0.1 to 10 parts of a lubricant, and (e) from 0 to 20 parts of a filler selected from calcium carbonate, clay, titanium oxide and carbon black.
Japanese Unexamined Patent Publication (Kokai) No. 43144/1980 discloses a vinyl chloride resin molding material obtained by blending a hard vinyl chloride resin composition with from 2 to 20 PHR of fine calcium carbonate treated with a fatty acid, from 0.3 to 3.0 PHR of at least either a magnesium oxide or a magnesium hydroxide, and from 0.2 to 2.0 PHR of at least either a dibutyltin dilaurate or a dibutyl-β-mercaptopropionate.
Japanese Unexamined Patent Publication (Kokai) No. 262941/1993 discloses a vinyl chloride resin composition obtained by blending a vinyl chloride resin with (a) at least one compound selected from the group consisting of magnesium oxide, magnesium carbonate and magnesium hydroxide, (b) a calcium oxide and/or a calcium hydroxide, (c) a zinc salt of an organic acid, and an organic phosphorus ester.
Japanese Unexamined Patent Publication (Kokai) No. 1898/1994 discloses a blending agent for chlorine-containing polymers, comprising a homogeneous composition of a crystalline layered magnesium phyllosilicate and a magnesium hydroxide or a magnesium oxide, the molar composition ratio of SiO2/MgO as a whole being in a range of from 0.25 to 1.33. In Comparative Example 1 of this publication, there has been described that a composition having an SiO2/MgO molar composition ratio of 0.10 fails to exhibit heat stability to a sufficient degree.
Japanese Unexamined Patent Publication (Kokai) No. 62182/1995 discloses a polyvinyl chloride resin composition comprising a polyvinyl chloride resin and a calcium hydroxide of a particle diameter of not larger than 40 μm.
Japanese Patent No. 2895108 discloses a composite stabilizer for chlorine-containing polymers, comprising from 0.5 to 1.7 parts by weight of a calcium hydroxide or a magnesium hydroxide, from 0.05 to 0.4 parts by weight of a metallic soap-type stabilizer and from 0.1 to 0.4 parts by weight of perchloric acid per part by weight of a zeolite-type stabilizer.
The calcium hydroxide and magnesium hydroxide are bases of typical divalent metals, and are presumed to be highly capable of trapping (reacting) hydrogen chloride per a unit weight. When really used as a stabilizer for the chlorine-containing polymers, however, they tend to initially color the chlorine-containing polymers to a warm color close to red and, besides, their heat stabilization time (time until the chlorine-containing polymer becomes blackened) is not as long as expected. Therefore, the calcium hydroxide and the magnesium hydroxide have simply been used as assistant components for the stabilizer as is done in the prior art.
The reason that the calcium hydroxide or magnesium hydroxide has a large tendency for imparting the initial color and does not have so long heat stabilization time is attributed to that the hydroxide is strongly basic and is rather inducing the reaction (polyene-forming reaction) for removing hydrogen chloride from the chlorine-containing polymer.